Peter decides to mix grades of gasoline in his truck. He puts in 5 gallons of regular and 10 gallons of premium for a total cost of $31.05 If premium gasoline costs $0.18 more per gallon than regular, what was the price of each grade of gasoline?

x = cost of 5 gallons of regular
x + 0.18 = cost of 10 gallons of premium{premium is $0.18 more than regular per gallon}